The use of drives and transmissions in mechanical and electromechanical systems is quite common for such applications as transportation, motion control, electronics, machine tools, printing machines, robotics and aerospace. When the load requirements are predictable, it is generally simple to design a drive or transmission system with single or multiple gear stages. The situation becomes more complex when the load requirements vary and are unpredictable. Usually in these circumstances, the systems are designed with maximum anticipated values such as maximum anticipated speed and maximum anticipated torque. As a wide range of torque and speeds need to be provided, the drive and transmission systems typically are larger in size, having more components and thereby increasing the cost.
Robotic systems are one example of a context where these issues arise. Robots are currently being used in numerous applications and in numerous ways. A few examples of their utilization may be found in industrial applications where robots are used to perform repetitious or strenuous tasks and in medical applications sometimes to assist a surgeon in performing surgery through teleoperation. For all their proliferation, robots are still limited in various ways.
One limitation which characterizes many robots is that they can only operate a certain amount of time with a given battery size. One cause of this limitation is due to the losses that occur in the process of converting electrical energy to mechanical outputs. Improving the efficiency of conversion would then directly lead to longer operation time with a given a battery size. In addition, the concepts to improve efficiency in mobile robots may be advantageous on stationary wall powered robots as well. For example, improved efficiency may lead to smaller motors thus leading to more compact design.
Yet another limitation that also characterizes many robots is that they are often designed for the worst case load or speed conditions, making them big and bulky. Designing a robot that can quickly adapt to changing load conditions and operate with high efficiency would be desirable. A desirable characteristic for many robotic systems is to have drive systems that achieve high gear reduction ratios. Typically, in conventional systems, high gear reduction ratios are achieved by addition of multiple gear stages. This adds weight to the system which consequently causes the inefficiencies to increase further which again contributes to lower operating range or duration in the case of mobile robots. Transmission systems with large gear reduction ratios in a single stage are available commercially. However, these systems still have the disadvantages of being expensive and needing precise manufacturing techniques.
Thus there is a need for efficient, light-weight, compact and fast actuating multispeed or variable transmissions which can be applied in actuators, manipulators, such as are used in robotics, and other devices to address the needs stated above. Multispeed or variable transmissions have been developed for a variety of applications but have not been widely used in robotics and other systems due to their complexity, volume and weight. Thus there is a need for single stage, efficient, light-weight, compact, low-cost drive systems which achieve high gear ratio reduction. More advantages may be obtained by making the transmissions variable.
In many applications, some as robots, since the speeds and torques of manipulators are highly cyclical and variable, using fixed gear ratio transmissions results in high energy inefficiencies because the motor is usually operating away from its optimum efficiency speed. It also limits the range of forces and speeds the manipulator can operate at, often forcing a designer to oversize the motors in order to meet worst case requirements. Just as a fixed-gear bicycle does not provide efficient locomotion in hilly terrain, a fixed-gear-ratio transmission actuator limits the capabilities of a robotic joint and wastes energy
Minimizing the time it takes to change gear ratios also leads to advantages since any delays would limit the speed at which the manipulator can change loads and as a result the range of tasks that it can accomplish. For example, for a walking robot, the leg needs to move fast and with no load during the swing phase, but slower and with a higher load during the stance phase of the stride. Similarly, during a pick and place operation, a manipulator placing a heavy load will need to switch between high load/low speed and low load/high speed quickly to enable minimizing the cycle time.
Other advantages may be realized by being able to shift gear ratios under load and at zero speed to avoid limiting the range of tasks an actuator or manipulator can perform. Many existing transmission designs can only shift when they are in movement or not under load